1. Field of the Invention
This invention relates to an image scanner, a facsimile transmission system and other image input systems, or image sensors of various types, and a photoelectric conversion device (or photoelectric transducer) used in them. More particularly, it relates to an improvement of pixel (picture element) structure.
2. Related Background Art
Photoelectric conversion devices are widely used in the above image input systems and image sensors.
As generally known examples, CCD image sensors making use of a reduction optical system and non-reduction contact type image sensors multiple-mounted with a plurality of semiconductor photosensor chips are used in, e.g., image scanners for inputting images. These are being developed energetically.
As photodetectors used as photoelectric conversion devices in these systems and sensors, it is common to use photodiodes comprising a semiconductor p-n junction. For example, as disclosed in Japanese Patent Application Laid-Open No. 55-159784, a structure is proposed in which a substrate having no p-n junction formed therein is provided, on its surface, with a region having the same conductivity type as the substrate and also having a higher impurity density than the substrate so that the dark current may less occur on the substrate surface. Also, as photodetectors for one-dimensional photoelectric conversion devices, proposed in variety are, e.g., a photodetector whose junction capacitance formed by p-n junction has been made small as disclosed in Japanese Patent Application Laid-Open No. 61-264758 and a photodetector in which the dark current ascribable to the scribed area at chip ends has been designed to less occur as disclosed in Japanese Patent Application Laid-Open No. 1-303752.
Meanwhile, as disclosed in, e.g., Japanese Patent Application Laid-Open No. 9-205588, an amplification type photoelectric conversion device is proposed in which a photodiode is used as a photodetector and electric charges of this photodetector are full read by using a source follower amplifier.
In the case of the above amplification type photoelectric conversion device, its light output Vp is expressed by Equation (1).Vp=Qp/Co×G   (1)where Qp is a quantity of electric charges accumulated at the p-n junction, Co is a capacitance at the photoelectric conversion area, G is an amplification degree determined by source follower gain, capacitance-dividing gain, amplifying gain and so forth.
In the case of an amplification type photoelectric conversion device having, e.g., a photodiode, an MOS source follower transistor and a reset MOS transistor, this capacitance Co at the photoelectric conversion area can be expressed by:Co=Cpd+Ca   (2)where Cpd is a p-n junction capacitance of a p-n photodiode, and Ca is other capacitance connected to the photoelectric conversion area, which includes, in the above case, source/well junction capacitance of the reset MOS transistor, source/gate overlap capacitance, wiring capacitance and the like.
Accordingly, in order to achieve a high sensitivity, it is important to accumulate photoelectric carriers (carriers formed upon exposure to light) effectively and make as small as possible the capacitance at the photoelectric conversion area at which the carriers are accumulated.
In general, the reset noise produced when a photodetector is reset increases with a decrease in capacitance at the photoelectric conversion area. However, as disclosed in the above Japanese Patent Application Laid-Open No. 9-205588, the reset noise can be eliminated by providing a means for retaining noise signals occurring immediately after resetting, and subtracting noise signals from light signals.
However, in such a photoelectric conversion device in which the noise signals occurring immediately after the resetting of a photodetector are retained during accumulation and subtracted from light signals to eliminate the reset noise of the photodetector, some external noises radiated from the outside of the photoelectric conversion device, in particular, low-frequency noises such as power-source noise may lower image quality.
Now, where the capacitance at the photoelectric conversion area is represented by Co, the amplification degree by G and the quantity of photoelectric charges accumulated at the photoelectric conversion area during accumulation by QP, noise output VN immediately after the resetting of a photodetector, the quantity of noise charges by QN and light output VS after accumulation are expressed by:VN=(√{square root over (kTCo)}/Co)×G ≡(QN/Co)×G   (3)andVS=((QN+QP)/Co)×G   (4),respectively, and the subtraction made between Equations (3) and (4) gives the following equation as light signals VP.
                              V          P                =                                            V              S                        -                          V              N                                =                                                    (                                                      (                                                                  Q                        P                                            +                                              Q                        N                                            -                                              Q                        N                                                              )                                    /                                      C                    O                                                  )                            ×              G                        =                                          (                                                      Q                    P                                    /                                      C                    O                                                  )                            ×              G                                                          (        5        )            
However, once, e.g., a low-frequency noise of about 50 Hz radiated from a power source of an instrument has mixed into a photodetector, it becomes impossible to eliminate the noise by the subtraction. Especially in the case of the above prior art, the timing at which the noise signals and light signals for which the subtraction is to be made has deviated by the time substantially corresponding to the time of accumulation and also the node of the photodetector during accumulation has come to have a very high impedance in the state of floating, and hence this external noise brings about a very great influence.
Now, where external-noise voltage present in the photodetector at a time (t1) where the noise signals occurring immediately after resetting are read is represented by VN1, and external-noise electric charges present in the photodetector at a time (t2) where the light output is read after accumulation by VN2, light signals VP′ after the subtraction corresponding to the above Equation (5) is as follows:VP′=((QP/Co)+VN2−VN1)×G   (6).
Also when an external noise having an amplitude VNR and a frequency fo (Hz) is radiated to the photodetector and as long as the noise voltage in the node of the photodetector is ΔVNR, the noise voltage at the photodetector node at an arbitrary time t is expressed by:ΔVNR(t)=ΔVNR×sin(2πfo×t)   (7).
Now, where the amplitude of external noise in the photodetector, ΔVNR, is set at 2±1 (an arbitrary unit), the frequency (fo) of the external noise at 50 Hz and the accumulation time of the photoelectric conversion device at 2.5 msec;    noise signal read time: tn (msec);    external noise at the time: tn: VN1;    time at the reading of light signals: ts (msec);    external noise at the time ts: VN2; and    subtraction of external noise: ΔVN(=VN2−VN1) at each field are given as shown in Table 1.
TABLE 1FieldnumbertnVN1tsVN2ΔVN1002.50.7070.70722.50.7075.01.00.29335.01.07.50.707−0.29347.50.70710.00−0.707510.0012.5−0.707−0.707612.5−0.70715.0−1.0−0.293715.0−1.017.5−0.7070.293817.5−0.70720.000.707920.0022.50.7070.7071022.50.70725.01.00.2931125.01.027.50.707−0.2931227.50.70730.00−0.7071330.0032.5−0.707−0.7071432.5−0.70735.0−1−0.293
As can be seen from the above table, mutual interferences between noise frequency and accumulation time cause periodic changes in output after subtraction.
Hence, in the case where e.g., all pixels of a photoelectric conversion device are read simultaneously, the above changes in output after the subtraction correspond to changes in the amount of offset in the direction of secondary scanning. Stated specifically, they appear as periodic stripelike light and shade in images thus read.